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1999 Chevrolet Suburban Misfire

1999 CHEVROLET SUBURBAN
215,000 MILES • 5.7L • V8 • 4WD • AUTOMATIC
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RASLINMASTER
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The PCV runs from the valve cover to (I have no idea) a black thing that has a vacuum tube running out of the top. The vacuum hose coming out of the top is broke off flush and I cannot for the life of me figure out where it goes. It has a lot of vacuum and when I put my finger over it the truck shuts off. My original issue is the truck has been running badly. I had a scan run and says number five cylinder misfire. I have recently replaced plugs, wires cap and rotor. I have good spark on number five and a brand new plug. So in my mind it must be a fuel issue. This morning i decided to put a new injector in the number five cylinder. In the process of taking off the upper intake I broke the cooling line off in the intake, which took a while to fix with out damaging the threads. I ended up fixing a couple other dry rotted lines but have two I am struggling with. By the way, the truck is running extremely rough so I do not know if the injector solved anything but I need to fix the vacuum issues so I can continue the illusive misfire saga. My Monday would really great if I could learn something useful.
Sep 10, 2017 at 3:30 PM
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STRAILER
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It would be great if you could upload a picture of what you are talking about? I have this diagram here (below) it should help.

Please let us know what happens.

Cheers, Ken
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Sep 11, 2017 at 6:38 AM
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RASLINMASTER
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I figured out the vacuum lines I think but still have random misfire code. Could a bad timing chain cause this problem?
Sep 13, 2017 at 2:20 AM
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STRAILER
  • ✓Experienced Mechanic
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A random misfire code can be caused by the MAF sensor. What happens is the sensor wire become dirty which alters the reading to the computer in turn leaning the engine down slightly causing the misfire.

Please follow this guide to clean the sensor and clear the codes to recheck the system.

https://www.2carpros.com/articles/mass-air-flow-service

Sometimes that doesn't work and you will need to replace the sensor.

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

and

https://www.2carpros.com/articles/check-engine-light-clear-codes

Please let us know what you find.

Cheers, Ken
Sep 13, 2017 at 4:04 AM
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RASLINMASTER
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It isn't throwing a code for the MAF
Sep 13, 2017 at 4:08 AM
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STRAILER
  • ✓Experienced Mechanic
  • 55,710 POSTS
It wont because there is not a non function problem, it could be just a little off. This is why many shops have a test MAF in stock at all times. The correct way is to take a live reading at idle with the used unit and then install a new unit to see if there is any difference.

Let me know
Sep 13, 2017 at 5:35 AM
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ROBERT BRADSHAW
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Need a picture of where the vacuum lines are routed.
Jan 27, 2021 at 3:40 AM (Merged)
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HMAC300
  • ✓Experienced Mechanic
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Hello,

There is the engine vacuum line diagrams as requested. Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.
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Jan 27, 2021 at 3:40 AM (Merged)
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RENEE L
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Hi Robert,

Thank you visiting 2CarPros. If you need further assistance please come back to the site. We are here to help.

Regards,

Renee
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Jan 27, 2021 at 3:40 AM (Merged)
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RUSSELL PENDLETON
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I need to see a diagram of the vacuum lines.
Jan 27, 2021 at 3:40 AM (Merged)
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DANNY L
  • ✓Experienced Mechanic
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Hello, I'm Danny.

I've attached 2 pictures below of vacuum diagrams for your truck. Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed. Hope this helps and thanks for using 2CarPros.

Danny-
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Jan 27, 2021 at 3:40 AM (Merged)
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DIREWOLF5699
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Dec 14, 2022 at 3:02 AM

Suggested Repair

Repair the broken vacuum lines using the vacuum diagrams. For the random misfire, clean the MAF sensor, clear codes, and recheck. If it continues, compare live idle readings with a known-good MAF and replace the sensor if the readings differ.

Conversation Summary

A 1999 Chevrolet Suburban was running rough with a cylinder 5 misfire after plugs, wires, cap, rotor, and an injector were replaced. A broken vacuum hose was also found while the upper intake was off. The owner later sorted the vacuum lines but still had a random misfire and asked whether a timing chain could cause it. The expert said a dirty or slightly inaccurate MAF can lean the mixture and cause misfires without setting a MAF code, and recommended cleaning it, clearing codes, then comparing live idle readings with a known-good sensor and replacing it if needed. Other replies supplied vacuum line diagrams.

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